Conventionally, there has been a known open winding type motor in which stator winding wires of respective phases are independent winding wires independent from each other. While a drive device for the open winding type motor may increase output capacity, a ripple of 0-axis current is a particular problem. To cope with such a problem, a technology referred to as zero common mode modulation (ZCMM) has been proposed in NPL 1.
When a power supply voltage is set to Ed, a voltage applied to each winding wire of the open winding type motor corresponds to three levels of {+Ed, 0, −Ed}, and voltages applied to three-phase winding wires of the motor correspond to 27 patterns. In the technology referred to as ZCMM described above, among space vectors obtained by converting the 27 patterns into αβ0-axis voltages, space vectors used for motor-applied voltages are restricted to seven space vectors at which a 0-axis motor-applied voltage corresponds to a zero value.
Here, a motor electric characteristic at a dq0-axis coordinate is expressed by Formula (1) below. A 0-axis inductance (Lz) is a function of dq-axis current. When the dq-axis current is set to a steady value, Lz is fixed, and an electrical characteristic of a 0-axis is not interfered by the dq-axis current. In Formula (1), Vd, Vq, and Vz denote motor-applied voltages of a d-axis, a q-axis, and the 0-axis, Id, Iq, and Iz denote a d-axis current, a q-axis current, and a 0-axis current, Ld, Lq, and Lz denote a d-axis inductance, a q-axis inductance, and a 0-axis inductance, r denotes a winding resistance, Ea denotes a fundamental wave induced voltage, Ez denotes a three-phase unbalanced component of an induced voltage, and P denotes a differential operator. Lz is an unbalanced component of an inductance, which is an extremely small value when compared to Ld and Lq.
                                              ⁢                  [                      Formula            ⁢                                                  ⁢            1                    ]                                                                              (                                                                      V                  d                                                                                                      V                  q                                                                                                      V                  z                                                              )                =                                            (                                                                                          r                      +                                              P                        ·                                                  L                          d                                                                                                                                                                        -                        ω                                            ·                                              L                        q                                                                                                  0                                                                                                              ω                      ·                                              L                        d                                                                                                                        r                      +                                              P                        ·                                                  L                          q                                                                                                                          0                                                                                        0                                                        0                                                                              r                      +                                              P                        ·                                                                              L                            z                                                    ⁡                                                      (                                                          Id                              ,                              Iq                                                        )                                                                                                                                                          )                        ·                          (                                                                                          I                      d                                                                                                                                  I                      q                                                                                                                                  I                      z                                                                                  )                                +                      (                                                            0                                                                                                  E                    a                                                                                                                    E                    z                                                                        )                                              (        1        )            
In ZCMM described in NPL 1, an induced voltage of the motor is limited to the case of only a fundamental wave. That is, when Ez is zero at all times, and a motor-applied voltage (Vz) of the 0-axis is held at zero by a scheme of ZCMM, the 0-axis current becomes zero.